The present invention relates to an oxidation catalyst for internal combustion engines which contains platinum as a catalytically active component on a finely divided support material.
As is widely known, catalysts which contain mainly noble metals from the platinum group of the Periodic System of Elements as catalytically active components are used for the treatment of exhaust gases from internal combustion engines.
Among the broad range of available catalysts for this purpose, the oxidation catalysts, reduction catalysts and so-called three-way converters can be differentiated.
The exhaust gas from internal combustion engines contains essentially three harmful substances; namely, carbon monoxide, unburnt hydrocarbons and nitrogen oxides. The term "nitrogen oxides" refers to a mixture of different oxidation states of nitrogen. About 90 vol. % of this mixture consists of nitrogen monoxide. The remainder mainly consists of nitrogen dioxide. This is conventionally indicated by the notation "NOX".
Three-way converters are able simultaneously to convert all three harmful substances to the harmless compounds; i.e., carbon dioxide, water and nitrogen. The prerequisite for this is that the internal combustion engine is operated with an air/fuel ratio close to the stoichiometric air/fuel ratio. The stoichiometric air/fuel ratio for conventional engine fuels has the value 14.6 kg of air per kg of fuel. In the event of simultaneous conversion of the three harmful substances, carbon monoxide and hydrocarbons act as reducing agents for the nitrogen oxides. Three-way converters contain rhodium as an essential component, in addition to platinum and/or palladium. The rhodium contributes to the ability of the catalyst to reduce nitrogen oxides.
Oxidation catalysts contain substantially platinum and/or palladium as catalytically active components. They can efficiently oxidize the unburnt exhaust gas components in oxygen-rich exhaust gas and thus convert them into harmless substances.
The catalytic activity of noble metal catalysts is extremely dependent on temperature. They are only slightly active at ambient temperature and therefore can only incompletely convert the harmful substances during the cold-start phase of internal combustion engines. The activity of the catalysts increases with increasing temperature. The light-off temperature of a catalyst is defined as the exhaust gas temperature at which the catalyst converts exactly 50% of a harmful substance into harmless gases. The light-off temperature of a catalyst may have a different value for each harmful substance. On achieving their operating temperature, which may be between 300 and 500.degree. C., depending on the type of catalyst, known catalysts convert harmful substances with an efficiency of more than 70%.
Modern diesel and petrol engines operate with a large excess of air, with air/fuel ratios of more than 18, in order to reduce the fuel requirement. In the partly loaded region, these engines have exhaust gas temperatures of less than 200.degree. C. To treat the exhaust gases from these engines, therefore, catalysts with as low as possible a light-off temperature and a high catalytic activity, even at temperatures below 200.degree. C., are required. This is currently achieved by using very large amounts of noble metal. Palladium is preferably used since it is less costly than platinum and it also has a higher oxidation activity.
An object of the present invention is to provide a catalyst which has a very low light-off temperature and a high catalytic activity when using a small amount of noble metal.